Television system



June 3, 1941' A. D. BLUMLEIN ETAL 2,244,239

TELEVISION SYSTEM Filled May 2a, 193s y 2 sheets-Sheet E 4' i Q Lf3/u g m f 7 1 H/l r /W l `/Lll gjml5 /g 22 (24T W l W www MMM INVENTOR ALAN 0.5mm /N RULF E. SPFNCER BY ATTORNEY June 3, 94- A. D. BLUMLx-:IN ET AL 2,244,239

TELEVISION SYSTEM Filed May 28, 1958 2 sheets-sheet 2 AAM VHF

INVENTOR L/V D. BLU/WIFI ROLFE. SPM/CFR ATTO R N EY Patented une 3, 1941 PATENTL GFFECE TELEVISIN SYSTEM of Great Britain Application May 28, 1938, Serial No. 210,576 In Great Britain June 7, 1937 4 Claims. (Q1. 1'78-7.1)

This invention relates to television transmission systems and has particular reference to such systems in which a carrier wave is modulated by mixed signals obtained from separate scanning cameras. employed, for example, in a system in which it is desired to superpose the matter to be transmitted upon a separate background or in which it is desired that the received picture shall contain a portion of a scene or object selected by one camera and also a portion of another scene or object selected by another camera.

In connection with systems employing superposed pictures a circuit arrangement has previously been proposed which permits a foreground scene to be superposed upon a background scene in such a manner that the background scene is obscured by the foreground scene. The circuit includes a normally operative camera for scanning the background, which camera is switched off during periods in which another camera scanning the foreground is' providing picture signals. The foreground scene is arranged in front of a black surface, the foreground camera being continuously operative and the background camera normally operative, superposes the scene it scans upon the black region of the foreground scene.

It is preferable to use a White surface behind the foreground scene, and to arrange that signals representative of the white surface are not transmitted from the foreground camera. Such an arrangement is preferable because in the former case it is not practicable to ensure that the whole surface of the foreground object is suiiiciently illuminated as not to produce a black area which would be treated by the separating circuit in the same Way as the black of the surface behind the foreground, with the result that the background scene will appear with the dark part of the foreground scene. In the case of a white surface behind the foreground scene, it can be arranged that this surface is always brighter (as by back projection) than any portion of the foreground scene, so that separation of the wanted and unwanted foreground signals can be effected.

When it is desired to change from one scene to another, it is the present practice to mix directly the outputs from the scanning cameras in a similar manner to changing from one microphone to another in sound transmission, and to fade one camera out as another is faded in. The effect of this is to allow one picture to dissolve into another equally over the whole surface of Two scanning cameras may be rthe screen at once. While this may be satisfactory in certain cases, it may give rise to confusion. Again, it does not lend itself to the case in which it is desired that portions of two pictures should be kept in View simultaneously without overlapping. In cinemafilms it is common for a line to move across the screen deleting one picture and replacing it by another, and this effect is obtained by splicing films or by optical means. In the case of an electric scanning camera the optical method is not in all cases applicable as different scenes may require different focusing, and of course there is no time for editing, asin the caseof films.

The object of the present invention is to provide a system of television transmission in which superposed pictures. are transmitted and received without confusion due to overlapping of the superposed pictures, and further, to provide a system in which a received picture shall contain portions of scenes selected by different scanning cameras. y

According to the present invention a television transmission system includes two scanning channels one of which contains signals representative of a foreground scene, with a bright rear surface which may be White and the second channel containing signals representative of a background scene and wherein means are p rovided for selecting from the said first channel signals which are representative of the light value of the rear surf-ace which may be effectively the peak white value, further means being provided fordeveloping from the selected signals two control signals one of which is effective to control the background channel and the other of which is effective to control the foreground channel'in such` a manner that the foreground channel is operative and the background channel is inoperative in the absence of said peak or white signals in the foreground channel and vice-versa, the outputs of said foreground and background channels being fed to a common channel.

In a particular transmission system embodying the invention the duration of the peak or white signals selected from the foreground channel is lengthened before the signals are used to develop control signals which effect switching in the channels. The lengthening of the selected signals may be effected by applying them to a delay network, taking from the delay network two tappings spaced apart by a predetermined amount, combining the output from the spaced tappings, said output being subsequently fed to an amplitude limiting device followed by an ampliiier, the output from which constitutes said control signals which are then applied in opposed phase to the foreground and background channels. In such a system the signals in the foreground channel are delayed between the point of selection of the signals representative of the peak or white value and the point of application of the control signals so that the foreground channel is rendered operative slightly after a change from the peak or white value to the value of the foreground scene and is made inoperative slightly before the change of signals from the foreground value to the peak or white value. 'Ille function of the lengthening of the signal representing the peak white value of the control channel and the delay of the signals in the foreground channel ensures that small errors in timing, together with a finite time required for a change of signalling strength from that representing the peak white Value to the foreground Value or vice-versa, do not cause white edges to appear around the foreground scene due to the peak white signals representing the white rear surface.

The signals from the foreground and background channels may be further controlled by a switching device such as a thermionic valve to a biased control electrode of which a voltage waveform of selected shape is applied in order that the output from the foreground and backaol ground channels may be combined in a predetermined manner. The waveform applied to the thermionic valve may be a line or frame frequency saw-tooth defiecting waveform in which case the picture area resulting from the combined channels will be divided by a moving vertical or horizontal line respectively. Instead of such a waveform, the output from a third scanning channel may be applied to the thermionic valve constituting the switching device.

In order that the invention may be more clearly understood and readily carried into effect, parts of a television transmission system embodying the invention Will now be described by way of example with reference to the drawings in which:

Figure 1 is a block diagram illustrating one method of carrying out the invention, and

Figure 2 is a circuit diagram illustrating one method of reproducing pictures containing separate portions of different transmitted scenes of objects.

Referring to the drawings, I is the input terminal to the foreground channel, 2 is the input terminal to the background channel, and 3 is the tapping point from the foreground :channel to the control channel. One line of an incoming signal is represented by the waveform 4 and consists of a peak white pulse, a vision signal and a second peak white pulse, the vision signal corresponding with an object in the foreground scene and extending over about the middle third of the line. The black and white levels are indicated at 5 and 6 respectively. One line of the background signals is represented by the waveform l and consists of continuous vision signals representing the background, and for the purposes of explanation the effective height of the vision signals is shown to be the same in the foreground and background channels.

The peak white signal pulses from the foreground channel are selected by a lter represented by the rectangle 8 and are passed to a delay network represented by the rectangle V9 giving a total delay of 2 n, the interval n being 3,7 senting the black signal.

indicated on the signals represented at IU. Tappings on the delay network 9 give outputs of delay by 1/2 n and 11/2 11. The resultant outputs of the network 9 are fed to a mixer II and then to a limiter I2 to produce a control signal I3 which has peak white pulses each longer by the period n than the original peak white pulses. The signals I3 are converted to push-pull signals by the stage I4 and applied in different phase to the foreground and background channels.

The signals in :both these channels have been delayed by networks I5 and I6 respectively, by a period n and have been amplified at I1 and I8 respectively, to increase the strength of the vision signals. In the foreground channel signals have also been limited at il to remove some of the surplus of the peak white, so as to prevent overloading of subsequent apparatus.

It will be seen that the signals in the background channel are now delayed by an amount lying between the maximum and minimum delay used for the control channel, so that the control signals overlap at their beginning and end of the peak white signals in the foreground channel. The delay to amplification of the background channel is included to retain symmetry of the apparatus, and is not necessary for the correct functioning of the circuit. The control signals I9 are then fed to a mixer 2D in the foreground channel in such a phase as to drive the peak white signals in the foreground channel in the black direction beyond the amplitude repre- Similarly the inverse control signals 2| are applied to a mixer 22 in the background channel and drive the signals in that channel beyond the black amplitude during the period that the foreground channel is not driven black. The signals in each channel are then applied to limiters 23 and 24 in which they are limited to an amplitude representative of black amplitude. The 'signals from both channels are then fed to a. mixer 25 in which they are mixed to give a complete picture represented by the waveform 26 containing both foreground and background signals.

It will be seen that the mixing of the control signals with the channel signals and the subsequent limiting is in reality only a method of switching these channels on and of and if preferred some device such as an hexode switching valve may be employed instead.

It will be noted that there is an overall unused delay of and this could be avoided by reducing the delay in the foreground and ,background channels to and reducing the delay in the control channel to n and taking olf the signals from the beginning and end of the delay network. The same result would be produced but this method would not allow any quick adjustment in hand. It is preferred to arrange for an excessive delay in the main channels so that the amount of overlapping of the control signals can be adjusted by movable tappings, the tappings in this case being half-way between minimum and maximum.

The arrangement described with reference to the drawings does not allow for any difference of delay inthe channels whichcan, of course, be compensated for in the delay networks. In the absence of any delay a slight white Whisker would occur at one or both sides of the foreground object due to the fact that all signals are slightly rounded. and in selecting the peak white signals the topmost and so the shortest portion of these signals is selected. The periods of delay are adjusted so as to remove any white edges around the foreground objects. Adjustment of the delay period to a value greater than that necessary to eliminate the white edges will tend to encroach on the correct'shape of the foreground object. It is advisable to develop very sharp signals from the push-pull stage It and to preserve as much as possible of this sharpness in the mixer circuits so that the remnant portions of the two channels fit as accurately as possible. The frequency range of the limiter circuit l2 in the control channel, the push-pull amplifier I4 and the mixers 2i) and 22 may with advantage be of the order of twice that of the general channels preceding and following.

The system described can be used for other purposes than superposition. Thus by feeding square topped signals into the limiter circuit l2 in the control channel the channels cari' be switched on and off so that part of the observed picture comes from one channel and part from another. Line frequency signals will produce vertical division of the final picture and frame frequency signals will produce horizontal division. A combination of line and frame signals will produce a line of change traversing the picture at any desired angle. A convenient method of obtaining such signals for producing a fading effect from one picture to another by a moving line across the received picture, is to provide a limiter having considerable amplification in the control channel, so that given a saw-tooth input it will by effect produce a stepped Wave. If a saw-tooth input at line frequency is then added to a variable D. C. bias potential, a vertical change-over can be made from one channel to another, the position of the change-over varying With the bias potential. Such a limiter can conven'iently be made of a number of stages of pushpull amplification, direct current coupled as described in the specification of British Patent No. 482,740.

The amount of direct current control required to operate the complete picture change will be the same as the amplitude of the saw-tooth waveform applied. As the latter may be controllable to give varying width of overlap the direct current available for effecting the changeover is preferably controlled by the same adjusting means as that controlling the saw-tooth amplitude. A further control may also be provided to effect rotation of the angle of the line of overlap without altering the sum of the line and frame saw-tooth waveforms or the direct current applied to the picture change control itself.

Again various types of signal, together with a direct current component having different waveforms niay be injected at the position of the limiter circuit l2 so as to produce different types of fading effect from one picture to another. Thus, if a sine wave is used to effect the picture change instead of a saw-tooth waveform, a new picture can be faded up into an existing picture starting from a vertical line in the center. Again, a combination of line and frame frequency sine waveforms would enable a new picture to appear first as a spot in the center of the old picture and to open up as an ellipse or a circle depending upon the relative amplitudes of the sine waves. Other effects may be obtained with the use of multiple frequency saw-tooth or sine waves.

Referring now to Figure 2 of the drawings, a particular form of circuit arrangement for obtainingvarious fading-in effects is shown. The valves 26 and 21 are the final amplifier valves of two separate channels previous to mixing. The output is taken from the anodes of the valves 2t and 2l which are connected together. Further valves 2B and 29, the anodes of which are connected to the line connecting the anodes of the valves 26 and 21, are driven in push-pull from another pair of valves 30 and 3| which operate as push-pull driving valves in a similar manner to that described in the specification of British Patent No. 482,740 with reference to Figure 3 of the drawings accompanying that specification. The grid of the valve 30 is fed from a source of controlling potentials which may, for example, be of any desired waveform, and the output from the Valves 'B and 3l supplies push-pull limited voltages to the valves 23 and 2S. The point at which the picture changeover is effected is determined by a bias potential applied to the grid of valve 35, the value of said bias potential being adjustable. The valves 28 and 2S serve to render the valves 26 and 2 operative alternatively, so that signals from the channel connected to the valve 28, or signals from theV channel connected to the valve 2, pass to the common output alternatively. A condition of direct current balance of the anodes of the valves which are connected together between the two conditions can be obtained by a variable bias potential applied to one or other of the valves.

The arrangement described with reference to Figure l may be used in conjunction with a third camera channel, the third camera being connected to the input of the limiter circuit l2 in the control channel. By placing a silhouette vin front of the third camera the background channel may be switched into operation for a portion of the picture not covered by the silhouette and the foreground channel for the portion of the picture covered by the silhouette or vice versa. In similar manner variable objects may be placed in front of the third camera so that variable superposition effects of the two channels are obtained. Thus a fadeover from one picture to another may be made by scattering ink on' a white surface in front of the kthird camera, the new picture channel being switched into operation at the points made black by the ink dots.

In the case of the circuits described it is very advantageous that the direct current component of the picture signals be present at every limiting or mixing operation, since the amount of signal representing peak white in the foreground picture is dependent upon the width of the foreground object and if this width varies, unless the direct current component is present, it will be impossible to select the peak white pulses with any certainty. Similarly the mixed peak white pulses will vary in length, and unless there is correct direct current transmission through this channel the subsequent limiting operation will be aiected as also will the generation of the pulses of correct amplitude. The direct current transmission may be effected by employing a direct current coupled amplifier throughout or the direct current may be re-inserted by well-known means prior to any point of amplitude limitation,

Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that what we claim is:

l. In a composite television system wherein separate background and foreground scenes are provided, the foreground scene including a bright white background surface, means for producing a rst series of picture signals in accordance with the light values of the foreground scene and associated white background surface, means for simultaneously and synchronously producing a second series of picture signals in accordance with the background scene, means responsive to said iirst series of picture signals for producing a series of peak white signal impulses corresponding to the white background surface of the foreground scene, means including a delay network for developing from the produced series of peak white signal impulses two series of control signal impulses substantially 180 degrees out of phase with each other and of a length such that they overlap the peak white signal impulses by a predetermined slight amount, means responsive to one of said series of control signal impulses for suppressing that portion of the first series of picture signals corresponding to the bright white background of the foreground scene, means responsive to the other of said series of control signal impulses for suppressing that portion of the second series of picture signals corresponding to the foreground scene, and means to combine the unsuppressed portions of the first and second series of picture signals.

2. In a television system for producing composite picture images wherein separate foreground and background scenes are provided, the foreground scene being positioned before a bright white background surface, means for producing a first series of picture signals in accordance with the light values of the foreground scene and its associated white background surface,

means for simultaneously and synchronously producing a second series of picture signals in accordance with the light values of the background scene, means for developing from said first series of picture signals a series of control signal impulses in accordance with the presence of peak white signals in said first series corresponding to the bright white background surface of the foreground scene, means including a delay network for causing the control signal impulses to overlap the foreground scene signals by a slight amount, means whereby said control signal impulses are effective to prevent the transmission of the rst mentioned series of picture signals when peak white signals corresponding to the bright background surface are present, and

for simultaneously permitting the transmission of the second series of picture signals, and means for combining the transmitted portions of each series of picture signals to form a single composite series of picture signals representative of the composite foreground and background scenes.

3. A television system comprising a rst and a second amplifying channel each including a delay network, means for impressing a rst series of picture signals upon one of the channels representative of a foreground scene which may contain a predetermined percentage of peak white signals, means for impressing a second series of picture signals upon the second amplifying channel representative of a background scene, means including an additional delay network for producing control signal impulses in accordance with the presence of peak white signals in the rst series of picture signals, the delay networks operating to extend the duration of the control signal impulses such that they overlap the duration of the peak white signals by a slight amount, means responsive to the produced control signal impulses for preventing the transmission of all of the peak white signal portion of the rst series of picture signals through the said one amplifying channel and for simultaneously permitting the transmission of the second series of picture signals through the said second amplifying channel, and means to combine the picture signals transmitted through the amplifying channels to produce a composite series of picture signals.

4. A television system comprising a first and a second amplifying channel, means for impressing a first series of picture signals upon one of the channels representative of a foreground scene which may contain a predetermined percentage of peak white signals, means for impressing a second series of picture signals upon the second amplifying channel representative of a background scene, means for producing a signal impulse series in accordance with the presence of peak white signals in the first series of picture signals, means to increase the time duration of the impulses of the produced series by a predetermined small amount to produce control signal impulses, means responsive to the produced control signal impulses for preventing the transmission of the peak white signal portion of the first series of picture signals through the said one amplifying channel and means for simultaneously permitting the transmission of the second series of picture signals through the said second amplifying channel, and means to combine the picture signals transmitted through the amplifying channels to produce a composite series of picture signals.

ALAN DOWER. BLUMLEIN. ROLF EDMUND SPENCER. 

